1. Field of the Invention
The present invention relates to a multiple chamber wafer processing system and, more particularly, to a method and apparatus for managing scheduling in a multiple cluster tool.
2. Description of the Background Art
Semiconductor wafers are processed to produce integrated circuits using a plurality of sequential process steps. These steps are performed using a plurality of process chambers. An assemblage of process chambers served by a wafer transport robot is known as a multiple chamber semiconductor wafer processing tool or cluster tool. In a single cluster tool, wafers are moved from one chamber to the next by means of a transfer mechanism (one or more robots). This wafer movement is known as a wafer transfer. The transfer mechanism can only move wafers among chambers that are within the transfer mechanism""s xe2x80x9cspacexe2x80x9d. The set of chambers that are reachable by a given transfer mechanism are referred to as the transfer space or the robot space. A cluster tool may be comprised of one or more transfer spaces, i.e., one or more transfer mechanisms in different spaces having access to several chambers in each of the spaces.
When a wafer processing system is comprised of two or more transfer spaces, the system is referred to as a multiple cluster tool or a multi-cluster tool. In a multi-cluster tool, wafers visit chambers from different transfer spaces and thus they are moved both within transfer spaces as well as between different (adjacent) transfer spaces. Wafers may be transferred from one space to another via a common chamber that is accessible from adjacent transfer spaces. The chamber that forms a connection between adjacent transfer spaces is known as a pass-through chamber. The pass-through chamber may also perform wafer processing.
Suppose that a pass-through chamber A connects transfer spaces R1 and R2. If chamber A is the only pass-through chamber, then chamber A must facilitate a xe2x80x9cdouble-passxe2x80x9d, i.e., chamber A is used to transfer the wafer in both directions between transfer spaces R1 and R2. If there is at least one more pass-through chamber, then chamber A may be either single-pass (one direction) or double-pass (bi-directional) type of chamber. Furthermore, a pass-through chamber may have either single or multiple capacity (i.e., the chamber could hold one or more wafers).
Pass-through chambers may severely limit a tool""s throughput in that the pass-through chamber may form a bottleneck with respect to wafers being transferred from one transfer space to another. An example of a system that has limited throughput is a multi-cluster tool system that has only a single capacity pass-through chamber. Such a system requires that one transfer mechanism in one of the transfer spaces wait for access to the transfer chamber while the other transfer mechanism accesses the transfer chamber. Another example of a system that has limited throughput because of the transfer chamber is a system where the pass-through chambers have long processing times and/or frequent cleaning processes. Thus, to avoid making the pass-through chamber a bottleneck and to achieve optimal throughput in a multi-cluster tool, management of the pass-through chambers (i.e., which wafer enters/leaves and when) is considered when designing scheduling logic for multi-cluster tools.
As an illustration of a multi-cluster tool, FIG. 1 depicts a schematic diagram of an illustrative multiple chamber semiconductor wafer processing tool known as the Endura(copyright) System manufactured by Applied Materials, Inc. of Santa Clara, Calif. This multi-cluster tool 100 comprises, for example, a preclean chamber 114, a buffer chamber 116 (a first transfer space R1), a wafer orienter/degas chamber 118, a cooldown chamber 102, four process chambers 104, 106, 108, 110, a transfer chamber 112 (a second transfer space R2), and a pair of loadlock chambers 120 and 122. The buffer chamber 116 is centrally located with respect to the loadlock chambers 120 and 122, the wafer orienter/degas chamber 118, the preclean chamber 114 and the cooldown chamber 102. To effectuate wafer transfer amongst these chambers, the buffer chamber 116 contains a first robotic wafer transfer mechanism 124. A collection of wafers 128 is typically carried from a previous location (storage or other tools) to the system in a plastic transport cassette 126 that is placed within one of the loadlock chambers 120 or 122. The first robotic wafer transport mechanism 124 transports wafers from collection 128, one at a time, from the cassette 126 to a designated chamber of the three chambers 118, 102, or 114. Typically, a given wafer is first placed in the wafer orienter/degas chamber 118, then moved to the preclean chamber 114. The cooldown chamber 102 is generally not used until after the wafer is processed within the process chambers 104, 106, 108, 110. Individual wafers are carried upon a wafer transport blade 130 that is located at the distal end of the first robotic mechanism 124. The transport operation is controlled by a sequencer 136.
The transfer chamber 112 (transfer space R2) is surrounded by and has access to the four process chambers 104, 106, 108 and 110 as well as the preclean chamber 114 and the cooldown chamber 102. The preclean chamber 114 and the cool down chamber 102 form the pass-through chambers that couple one transfer space R1 to another transfer space R2. The pass-through chambers are described here as uni-directional in that the preclean chamber 114 is used to move wafers into the transfer chamber 112 and the cooldown chamber 102 is used to move wafers out of the transfer chamber 112. However, these transfer chambers can be bi-directional.
To effectuate transport of a wafer amongst the chambers, the transfer chamber 112 contains a second robotic transport mechanism 132. The mechanism 132 has a wafer transport blade 134 attached to its distal end for carrying the individual wafers. In operation, the wafer transport blade 134 of the second transport mechanism 132 retrieves a wafer from the preclean chamber 114 and carries that wafer to a first stage of processing, for example, a physical vapor deposition (PVD) process within chamber 104. Once the wafer is processed (e.g., the PVD process deposits material upon the wafer), the wafer can then be moved to a subsequent stage of processing.
Once required processing is completed within the process chambers 104, 106, 108, and 110, the transport mechanism 132 removes the wafer from the last process chamber and transports the wafer to the cooldown chamber 102. The wafer is then removed from the cooldown chamber 102 using the first transport mechanism 124 within the buffer chamber 116. Lastly, the wafer is placed in the transport cassette 126 within the loadlock chamber 122.
To ensure an optimal schedule that facilitates a high throughput, a priority-based scheduling routine may be executed by the sequencer 136. The routine prioritizes the chambers within the cluster tool and computes the optimal schedule for movement of each wafer such that the wafer is fully processed in a minimal amount of time. Empirical testing to determine an optimal schedule is both expensive and time consuming. Present schedule development consists of having to xe2x80x9chard codexe2x80x9d each possible scheduling algorithm into the scheduler. Hard coding requires each scheduling algorithm to be individually considered and tested. The number of priority based routine possibilities that can be used to process a given wafer is staggering. For example, for a 5-stage cluster tool there would be at least 5!=120 different scheduling routines (each routine would have to be independently designed, developed and tested). This number of scheduling routines is for a single cluster tool and does not consider the impact of pass through chambers that connect the individual cluster tools.
Therefore, a need exists in the art for a method and apparatus for managing scheduling in a multi-cluster tool.
The disadvantages associated with the prior art are overcome by the invention of a method and apparatus for managing schedules for a multiple cluster tool. Through a graphical interface, a user selects a robot type and a concomitant scheduling algorithm. The scheduling algorithm, robot type, and desired processing information are made available to a multi-cluster tool simulator that simulates movements, processing and events which affect the wafers. Upon an event in the tool simulator (e.g., a chamber completes processing) or a trigger (e.g., a chamber enters or exists a cleaning procedure), the selected scheduling algorithm re-evaluates the wafer positions, i.e., the algorithm moves the wafers that can be moved and the simulation continues. The inventive method and apparatus computes a performance results (e.g., throughput, cycle time and the like). Once the simulation has xe2x80x9cprocessedxe2x80x9d all the wafers, the parameters used in scheduling the wafer movements through the simulated tool are modified and the process is executed again to obtain additional performance results. The results obtained for various scheduling algorithms can be compared and the best algorithm selected for use in a xe2x80x9crealxe2x80x9d multi-cluster tool.
The method and apparatus of the invention permit selection of any one of a large number of scheduling algorithms. By using an On-Line Priority Assigned Scheduling (OLPAS) algorithm, the invention analyzes N! priority based schedules (also known as scheduling algorithms) for an N stage cluster tool and then compares the results of each of the schedules.
To determine the best schedule for a multi-cluster tool, the On-Line Priority Assigned Scheduling (OLPAS) process is used to schedule wafer movement in each transfer space of a multi-cluster tool and a pass-through management process is used to manage one or more pass-through chambers that couple the transfer spaces to one another. Various embodiments of the invention enable schedules to be generated and analyzed for multi-cluster tools having single blade robots, dual blade robots, multiple pass-though chambers between transfer spaces, clean and paste procedures, and the like.